There's no official Lenovo shop here (but there is in Thailand and Vietnam) so you'll need to ring around the shops that stock Lenovo and hope you get lucky.

The last time I needed to do this, I took the laptop down Kampuchea Krom stopping at every second-hand laptop shop trying charges until we found on with the right plug, power and polarity. A bit of a chore but the job was done in a couple of hours.

You should be able to get them from Davy Computer, not far from Wat Phnom. They may not have them in stock, but they should be able to order in anything for computers.